Method of implanting a substrate and an ion implanter for performing the method

ABSTRACT

An implanter provides two-dimensional scanning of a substrate relative to an implant beam so that the beam draws a raster of scan lines on the substrate. The beam current is measured at turnaround points off the substrate and the current value is used to control the subsequent fast scan speed so as to compensate for the effect of any variation in beam current on dose uniformity in the slow scan direction. The scanning may produce a raster of non-intersecting uniformly spaced parallel scan lines and the spacing between the lines is selected to ensure appropriate dose uniformity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-In-Part of copending application Ser.No. 11/417,028, filed on May 4, 2006, and for which priority is claimedunder 35 U.S.C. § 120, which is a Continuation of application Ser. No.10/754,502, filed on Jan. 12, 2004, (now U.S. Pat. No. 7,049,210 dated23 May 2006) which is a Continuation-In-Part of application Ser. No.10/251,780, filed on Sep. 23, 2002 (now U.S. Pat. No. 6,908,836 datedJun. 21, 2005), and for each of which priority is also claimed under 35U.S.C. § 120. The entire contents of these applications are herebyincorporated by reference.

FIELD OF THE INVENTION

This invention relates to a method of implanting a substrate in whichthe relative movement between the substrate and an implant beam iscontrolled to maintain a desired and uniform dose of implanted speciesover the surface of the substrate. The invention also relates to ionimplanters adapted for performing the method.

BACKGROUND OF THE INVENTION

In a typical ion implanter, a relatively small cross-section beam ofions containing a desired atomic species is scanned relative to asubstrate to be implanted, typically a semi-conductor wafer.

The beam may be scanned transversely in two dimensions relative to thestationary wafer, or the wafer may be scanned in two dimensions relativeto a stationary beam. There are also hybrid scanning techniques when thebeam is scanned in one dimension whilst the wafer is mechanicallyscanned in a second typically orthogonal direction.

The various techniques have advantages and disadvantages. For batchprocessing of semi-conductor wafers, the wafers of a batch can bemounted on a rotating wheel and the axis of rotation of the wheel canthen be scanned to and fro to provide two-dimensional mechanicalscanning of the wafers across a stationary beam. An example of batchimplanter of this kind is described in U.S. Pat. No. 5,389,793.

Single wafer implanters can employ the hybrid mechanical andelectrostatic or electromagnetic beam scanning outlined above. Such anarrangement is described in our commonly assigned U.S. Pat. No.5,898,179. Here, the ion beam is electromagnetically scanned in a firstdirection perpendicular to the beam axis in the ion implanter, whilstthe wafer is mechanically moved in a second generally orthogonaldirection.

It is important in implanting to ensure that the total dose of desiredspecies implanted into the semi-conductor wafer or other substrate hasthe desired level of dose uniformity over the entire substrate surface.In the above-described batch-type of implanters, this is achieved byspinning the implant wheel at high speed and scanning the wheel axis toand fro so that the wafers mounted on the wheel pass across the beammany times during an implant process. In the hybrid single-waferimplanters also mentioned above, dose uniformity is maintained byperforming the electrostatic or electromagnetic beam scanning at arelatively high rate compared to the mechanical movement of the wafer tobe implanted. Dose uniformity over the wafer surface in the direction ofmechanical movement of the wafer is ensured by controlling the rate ofthis mechanical movement, but the rate of mechanical movement is alwaysmuch slower than the beam scanning rate.

SUMMARY OF THE INVENTION

It is an object of embodiments of this invention to provide novelscanning algorithms which can provide particular advantages as will beapparent from the following.

The invention provides a method of implanting a substrate wafer havingan implant surface with a predetermined diameter, the method comprising:

-   -   a) generating an implant beam having a predetermined beam        direction,    -   b) holding said substrate wafer aligned in an implant plane,        normal to said beam direction,    -   c) said beam having orthogonal cross-sectional dimensions in        said implant plane each of which is less than said predetermined        wafer diameter, and    -   d) mechanically scanning said substrate wafer in two dimensions        in said implant plane to provide a desired uniform total implant        dose over the implant surface of the substrate wafer,    -   wherein said mechanical scanning is performed so as to deliver        said total implant dose in first and second parts,    -   said first part being delivered by performing said mechanical        scanning with said substrate wafer held in a first orientation        about a twist axis parallel to said predetermined beam        direction, and    -   said second part being delivered to complete said total implant        dose by performing said mechanical scanning with the substrate        wafer held in a second orientation at 180° to said first        orientation about said twist axis.

By dividing the total implant dose into two parts a first delivered withthe wafer twisted by 180° relative to a second, a “dual” implant isobtained which can provide the advantages of a known multi-orientation(quad) implant, especially for zero angle implants using a fullymechanical (2-D) scanning implanter.

The invention also provides a method of implanting a substrate waferhaving an implant surface with a predetermined diameter, the methodcomprising:

-   -   a) generating an implant beam having a predetermined beam        direction,    -   b) holding said substrate wafer aligned in an implant plane,        normal to said beam direction,    -   c) said beam having orthogonal cross-sectional dimensions in        said implant plane each of which is less than said predetermined        wafer diameter, and    -   d) mechanically scanning said substrate wafer in two dimensions        in said implant plane to provide a desired uniform total implant        dose over the implant surface of the substrate wafer,    -   wherein the substrate wafer is mechanically scanned so that the        implant beam draws on the implant surface of the substrate wafer        alternating first and second rasters of scan lines, each of said        first and second rasters covering the whole of said implant        surface, said first rasters are drawn with said substrate wafer        held in a first orientation about a twist axis parallel to said        predetermined beam direction, and    -   said second rasters are drawn with the substrate wafer held in a        second orientation at 180° to said first orientation about said        twist axis. By ensuring the substrate wafer is twisted by 180°        between each 2-D mechanical scan pass (or raster), thermal        loading of the wafer during implanting can be made more uniform.

The invention further provides a method of implanting a substrate,comprising a) generating an implant beam having a measurable flux of adesired atomic species and b) producing relative movement between thesubstrate and the beam both (i) in a first direction transverse to thebeam direction to produce at least one pass of the beam over thesubstrate and (ii) in a second direction transverse to the beamdirection and said first direction to produce a plurality of scans ofthe beam over the substrate during each said pass, whereby said scansdraw on the substrate a raster of lines having mid-points which havepredetermined spacing in said first direction. The scans are arranged toextend beyond an edge of the substrate to positions at which no beamflux is absorbed by the substrate. The beam flux is measured at thesepositions and the speed of the next said scan over the substrate isadjusted in response to the previously-measured beam flux. The speedadjustment may be sufficient to compensate fully for beam currentchanges during the pass, so that the adjustment maintains a desired rateof implanting in the substrate of said atomic species per unit length ofthe scan over the substrate. However the speed adjustment may compensateonly partly, and other adjustments may be made as well, such as to thespacing between line mid-points and/or to the beam current itself.

In this method, the movement in the first direction can be called a slowscan between the beam and the substrate and the movement in the seconddirection can be called a fast scan. Preferably, the slow scan movementis maintained to ensure the raster lines drawn on the substrate by thefast scans have mid-points which are uniformly spaced. The amount ofdose per unit length delivered by each of the fast scans is thencontrolled from scan to scan in accordance with the measured beam fluxby adjusting the speed of the fast scans.

The invention may be employed using a continuous relative movementbetween beam and substrate in the slow scan direction, producing azig-zag or saw-tooth scan pattern on the substrate. However, betterresults are obtained if the scans draw a raster of non-intersectinguniformly spaced substantially parallel lines on the substrate.

Good dose uniformity over the substrate can then be obtained, even for asingle pass of the beam over the substrate in said first direction.

Importantly also, good dose uniformity is possible even if the fastscans of the beam in said second direction are themselves relativelyslow compared to the scanning rates achievable withelectromagnetic/electrostatic beam scanning systems. As a result, themethod has particular application in single wafer mechanical scanningsystems where the fast scanning of the wafer relative to the beam isachieved by a reciprocating mechanical movement of the wafer on a waferholder. With reciprocating mechanical scanning systems, the highest rateat which the wafer can be reciprocated across the implant beam islimited. As a result, with an implant beam having a predetermined fluxof the desired atomic species to be implanted, the amount of dosedelivered to each unit area of the substrate in the scan path, as thebeam makes a single traverse over that area, is much higher. Therefore,the spacing between successive fast scans, and in particular the spacingbetween the lines of the raster formed by the scanning system, tends tobe greater.

Controlling the speed of each fast scan in such a scanning system, inaccordance with the beam flux measured just before the scan, ensuresthat each fast scan delivers dose to the wafer at the same rate per unitlength of the scan so that the uniformly spaced scans deliver a uniformdose over the slow scan.

Preferably, said relative movement between the substrate and the beam insaid second direction is produced by mechanically scanning the substrateparallel to said second direction, and said relative movement in saidfirst direction is produced by mechanically translating said substrateparallel to said first direction by a uniform distance between scans.

The invention also provides a method of implanting a substratecomprising the steps of a) generating an implant beam having apredetermined beam flux of a desired atomic species, b) mechanicallytranslating the substrate parallel to a first direction transverse tothe beam direction to produce at least one pass of the beam over thesubstrate, c) mechanically reciprocating the substrate parallel to asecond direction, transverse to the beam direction and said firstdirection, to produce a plurality of scans of the beam over thesubstrate during each said pass, whereby said scans draw on thesubstrate a raster of lines having mid-points which have predeterminedspacing in said first direction, and d) controlling said mechanicaltranslating step to select said spacing of said mid-points of the rasterlines so that said raster provides a desired uniformity, in said firstdirection, of implanted dose of said atomic species over the substrate.

A two-dimensional mechanical scanning procedure of this kind enableswafers to be implanted singly (i.e. not in a batch process) using arelatively simple beam line without the beam scanning systems employedin hybrid scanning single-wafer implanters such as those discussedabove. Because reciprocating mechanical scanning of a wafer is likely tobe relatively slow compared to electrical beam scanning, relatively fewscans of the beam over the substrate are required to deliver to thesubstrate the dose of required atomic species specified by the processrecipe. This implies that the individual lines of the raster produced bythe scanning process may be spaced apart by a significant fraction ofthe width of the ion beam. Also the complete implant may require only asmall number of passes of the beam over the substrate (in theabove-mentioned first direction transverse to the mechanicalreciprocation direction, the second direction).

The method of the invention set out above ensures that the spacing ofthe raster lines is nevertheless such as to provide the desired doseuniformity over the substrate. Again, best results are obtained if thescans draw a raster of non-intersecting uniformly spaced substantiallyparallel lines in the substrate, which can be achieved by employing aplurality of translation steps in said first direction betweenrespective pairs of successive scans.

The controlling step set out above may include measuring thecross-sectional profile of the ion beam at least in said firstdirection, calculating from said profile a maximum value of said spacingof said mid-points which provides said desired uniformity, and adjustingthe mechanical translating step to select a value of said spacing whichdoes not exceed said maximum value.

Techniques for calculating from the measured profile the maximum valueof raster line spacing to provide the desired uniformity will beexplained in greater detail in the following description of examples ofthis invention.

Alternatively, or additionally, the controlling step may includemeasuring the cross-sectional profile as above, calculating a desiredvalue of said uniform raster line spacing from data including said beamflux, the speed of said mechanical reciprocation and the desired doseper unit area of substrate to be implanted, then using said measuredcross-sectional profile of the ion beam to calculate the dose uniformitywhich would be obtained at the calculated desired spacing value, andselecting said desired spacing value as the uniform spacing to beemployed, only if said calculated uniformity is not worse than saiddesired uniformity.

It will be understood that, given a predetermined beam flux of thedesired atomic species, and a known rate of scanning, the raster linespacing needed to provide the implant recipe dose to the substrateusing, for example, four passes of a substrate over the beam, can becalculated. However, this desired raster line spacing to achieve thedesired recipe dose can be used for implanting only if it results in thedesired dose uniformity. If the calculated uniformity is worse than thedesired uniformity, the beam flux and consequently the line spacing canbe reduced to a level at which the calculated dose uniformity is nolonger worse than the desired uniformity. Alternatively, or as well,steps may be taken to improve the spatial blending quality of themeasured beam profile, as will be explained later in greater detail.

In the summary above and the detailed description which follows, theterm “raster” is used to denote a set of scan lines drawn by an ion beamon a substrate being implanted. The scan lines of a single raster may bedrawn in a single pass of the beam over the wafer (in the slow scandirection), or may be drawn by two or more successive passes. It shouldalso be noted that a single scan line of the raster could be drawn bymultiple overlying scans of the beam over the substrate (in the fastscan direction).

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the invention will now be described with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic diagram of an ion implanter illustrating anembodiment of the present invention;

FIG. 2 is a schematic illustration of the mechanical scanning patternperformed by the implanter of FIG. 1 to produce a desired raster;

FIG. 3 illustrates an alternative scanning pattern which may beperformed;

FIG. 4 illustrates a further scanning pattern;

FIG. 5 is a graphical representation of an arbitrary ion beam intensityprofile function in one dimension;

FIG. 6 is an illustration of a semi-conductor wafer to be implantedshowing the scanning raster over the substrate, and illustrating anarbitrary beam shape;

FIG. 7 is a graphical representation of dose uniformity against thespatial frequency of the scanning raster lines on the substrate wafer,calculated by means of Fast Fourier Transform (FFT);

FIG. 8 is a similar graphical representation of dose uniformity againstspatial frequency showing additional points plotted by using the scalingproperty of a Fourier transform;

FIG. 9 is a further graphical representation of the dose uniformityagainst spatial frequency of raster lines calculated more precisely;

FIG. 10 is a graphical representation of an idealised beam profile inthe form of a Gaussian function;

FIG. 11 is a graphical representation of calculated dose uniformityagainst the spatial frequency of scanning raster lines for the Gaussianprofile of FIG. 10;

FIG. 12 illustrates a still further scanning pattern;

FIGS. 13 a to 13 d illustrate interleaved scanning rasters for a quadimplant;

FIG. 14 illustrates a profiled raster to improve productivity.

FIG. 15 is a graphical plot of the cross-sectional ion current profileof an ion beam;

FIG. 16 is a graphical plot of implant uniformity for interleavedrasters of scan lines against the pitch of the scan lines, and

FIG. 17 is a graphical plot similar to FIG. 16 showing the advantage ofoptimising the shift between the interleaved rasters.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, the illustrated ion implanter comprises an ionsource 10 from which a beam 11 of ions is extracted by means ofextraction electrodes 12. The ion beam 11 then passes through a massanalyser magnet 13 and ions in the beam emerging from the mass analysermagnet 13 which have a desired mass, corresponding to ions containingthe atomic species to be implanted, are selected from the beam by meansof a mass resolving slit 14. As is well known, the beam ions selectedfor implantation may be atomic ions containing only the atomic speciesrequired to be implanted, molecular ions including the required atomicspecies, or cluster ions comprising multiple atoms of the desiredspecies or multiple molecules.

These components of the ion implanter are standard and well known toworkers in the field of ion implantation. Together these components forman ion-beam generator which generates a beam of ions containing adesired atomic species for implantation in a semi-conductor wafer.

The mass selected ion beam from the beam generator described aboveenters an evacuated process chamber having a chamber wall of which partis illustrated at 15. A wafer scanning arrangement illustrated generallyat 16 is mounted on the wall 15 of the process chamber and is operableto scan a wafer 17 held on a wafer holder 18 in two directionstransversely of the mass selected ion beam 19. The wafer holder 18 ismounted at a distal end 20 of a scan arm 21 which extends through avacuum seal 22 from the interior to the exterior of the process chamber.The vacuum seal 22 is formed in a slide plate 23 which is mounted forlinear movement transversely of the longitudinal axis of the scan arm21, on a rotary carrier plate 24, which is in turn mounted for rotatablemovement in a plane transverse to the scan arm 21 relative to theprocess chamber wall 15.

The scan arm 21 is mounted for longitudinal movement on the slide plate23 and can be driven longitudinally to and fro by a motor 25. The slideplate 23 carrying the scan arm 21 can itself be driven transversely ofthe scan arm 21 by a drive motor 26. Suitable operation of the drivemotors 25 and 26 produce a combined scanning movement of the waferholder 18 and any semi-conductor wafer thereon in a two-dimensional scanpattern across the ion beam 19.

In a convenient example, the rotary support plate 24 of the scanningsystem is mounted on the process chamber wall 15 by an air bearing andvacuum seal combination such as disclosed in U.S. Pat. No. 5,898,179 andGB-A-2360332. Similarly, the slide plate 23 is mounted for linear motionon the rotary support plate 24 again by means of an air bearing andvacuum seal combination as disclosed in the above-referred U.S. patentspecification. The scan arm 21 is preferably mounted for longitudinalmotion through the vacuum seal 22 in the slide plate 23 by the linearmotor and compliant air bearing vacuum seal arrangement disclosed in ourInternational Patent Application WO 03/088303, the contents of thespecification of which are incorporated herein by reference in theirentirety.

In the process chamber of the implanter, a Faraday is located downstreamof the wafer holder 18 in a position to absorb the entire ion beam 19whenever the wafer holder 18 is positioned such that no part of the beam19 impinges on a wafer 17 on the holder 18, or on the scan arm 21. Thetotal charge absorbed from the ion beam by the Faraday 30 provides ameasure of the flux in the beam 19 of ions containing the atomic speciesto be implanted and hence of the flux of the desired atomic species. Acontroller 31 receives a signal from the Faraday 30 on a line 32, and isoperative to derive from this signal a value for the total beam flux ofthe desired species. The controller 31 is also operative to control theoperation of drive motors 25 and 26 so as to control the scanning motionof the scan arm 21.

In a preferred arrangement, the controller 31 is operative to move thewafer holder 18 in a sequence of linear movements across the beam 19 inthe plane of the paper of FIG. 1, with each linear movement separated bya stepwise movement normal to the plane of the paper. The resulting scanpattern is illustrated in FIG. 2 in which the dashed line 35 is thelocus of the centre 36 of wafer 17 as it is reciprocated to and fro bythe scan arm 21 in the X-coordinate direction, and indexed downwardly,parallel to the Y-coordinate direction, at the end of each stroke ofreciprocation.

In FIG. 2, the ion beam 19 is illustrated as having a substantiallycircular cross-section with a diameter which is substantially less thanthe diameter of the wafer 17. In practice, the wafer 17 may have adiameter of 300 mm, whereas the diameter of the ion beam is typically 50mm. As can be seen, the reciprocating scanning action of the wafer 17ensures that all parts of the wafer 17 are exposed to the ion beam 19.The movement of the wafer 17 causes the beam 19 to make repeated scansover the wafer 17 with the individual scans being parallel and equallyspaced apart, until the beam makes a full pass over the wafer.

Although the line 35 in FIG. 2 represents the motion of the wafer 17 onthe holder 18 relative to the stationary ion beam 19, the line 35 canalso represent a visualisation of the scans of the ion beam across thewafer. For this purpose, the wafer 17 is represented in dashed outline17 a at the centre of the illustrated scan pattern, and the ion beam isrepresented at 19 a. Obviously, the motion of the ion beam 19 a relativeto the wafer 17 a is in the reverse direction compared to the actualmotion of the wafer 17 relative to the ion beam 19.

In this example, the controller 31 scans the wafer 17 so that the ionbeam 19 a draws a raster of non-intersecting uniformly-spaced parallellines on the substrate. Each line 37 corresponds to a single scan of theion beam over the substrate. As illustrated, these ion beam scans extendbeyond an edge of the wafer 17 to positions 38 at which the beamcross-section is completely clear of the wafer 17 a so that no beam fluxis absorbed by the wafer. At these positions, the full flux of the beam19 a reaches the Faraday 30 located downstream of the wafer holder 18,so that the full beam current of desired species can then be determinedby the controller 31 from the signal on line 32. It will be appreciatedthat in the visualisation represented in FIG. 2 in which the beam 19 ascans over the wafer 17 a, Faraday 30 effectively moves with the beam 19a. In practice, of course, the beam 19 and Faraday 30 are fixed and itis the wafer 17 which is moved.

Assuming the beam flux of atomic species to be implanted is constantover time, the dose of the desired species delivered to the wafer 17 ais maintained constant over the wafer in the coordinate direction X ofthe raster lines 37 by maintaining a constant speed of movement of thewafer 17 along the raster lines. Also, by ensuring that the spacingbetween the individual raster lines 37 is uniform, the dose distributionalong the ordinate direction Y is also maintained substantiallyconstant.

In practice, however, there may be some progressive variation in thebeam flux during the time taken for the wafer 17 to perform a completepass over the ion beam 19, i.e. to complete the full raster of lines 37as illustrated in FIG. 2.

In order to reduce the effect of such beam flux variations during ascanning raster, the controller is arranged to measure the beam flux atthe positions 38 when the entire beam flux is absorbed by the Faraday 30and then use the measured flux to adjust the speed at which the waferholder 18 is moved over the next scan or raster line 37. If the beamflux measurement at the positions 38 indicates that the beam flux hasbecome less, then the controller ensures that the wafer holder is drivenalong the next scan line 37 at a slower speed so as to maintain adesired rate of implant of the required atomic species per unit distanceof travel of the wafer holder 18 along the scan line. In this way, anyvariations in the beam current during the course of the formation of acomplete raster of scan lines over the wafer does not result in avariation in the dose delivered to the substrate in the scan linespacing direction.

In the example described above, the wafer is maintained in a planeperpendicular to the ion beam during the scanning operation to completean implant. This is a zero angle implant. Angled implants can also beperformed, with the wafer (and more particularly the crystal structureof the wafer) maintained at a desired tilt angle and twist orientationto the implant beam. The wafer tilt angle is adjusted by rotating theentire scanning arrangement 16 on rotary support plate 24 about an axisparallel to the scan arm 21, in the plane of a wafer 17 on the waferholder 18, and intersecting the ion beam path 19. FIG. 1 is schematicand does not correctly show the geometry of the axis of rotation ofplate 24. The wafer twist orientation is adjusted by rotating waferholder 18, about a normal axis through the wafer centre, with a twistmotor 56 on the distal end 20 of the scan arm 21. Because the tilt angleof the wafer is adjusted by rotating the entire scanning arrangement 16,the plane in which the wafer is subsequently scanned is also tilted to aplane (X, Y¹) at the same angle relative to the ion beam. The tiltedwafer is then translated during a scanning raster in both X and Y¹directions in a plane parallel to the plane of the wafer.

In the example described so far, the relative movement between thesemi-conductor substrate or wafer and the ion beam is provided bymechanically scanning the semi-conductor wafer in two directionstransverse to the ion beam. This obviates the need to employ ion beamdeflection systems to scan the ion beam itself. However, in differentexamples, the beam may be scanned, by means of beam deflectors, in atleast one of the two transverse directions providing the two-dimensionalrelative scanning between the wafer and the beam. In all cases, however,the scanning system produces multiple scans of the beam over the waferin each of one or more passes, whereby the scans draw a raster ofnon-intersecting uniformly-spaced parallel lines on the substrate. Alsothe beam flux is measured at positions where the scans extend beyond theedge of the wafer and the speed of the next scan is adjusted to maintainthe dose rate per unit length of scan at a desired value.

In the mechanical scanning system described above with reference toFIGS. 1 and 2, the wafer 17 is translated by a uniform distance betweeneach individual scan, that is to say each individual stroke of thereciprocating motion of the scanning arm 21 along its axis, in order toproduce the zig-zag raster pattern illustrated in FIG. 2. However, thescanning mechanism may be controlled so that multiple scans areperformed along the same line of the raster. For example, each rasterline may represent a double stroke or reciprocation of the scanning arm21 and the wafer holder 18 is then translated by the uniform distanceonly between each double stroke. The resulting scanning pattern isillustrated in FIG. 3.

Also, FIG. 2 illustrates only a single pass of the beam over the waferparallel to the Y-coordinate direction, but the complete implantprocedure may include multiple passes. Then each such pass of theimplant process may be arranged to draw a respective independent rasterof uniformly-spaced parallel lines. However, the scan lines of themultiple passes may be combined to draw a composite raster effectivelydrawn from the scans of a plurality of passes. For example, the scans ofa second pass could be drawn precisely mid-way between the scans of thefirst pass to produce a composite raster having a uniform raster linespacing which is half the spacing between the successive scans of eachpass.

As mentioned previously, using a purely mechanical scanning systemnormally results in the maximum speed of travel of the ion beam over thesemi-conductor wafer being limited by the maximum speed of mechanicalscanning of the wafer holder. In a mechanical scanning system of thekind illustrated in FIG. 1 and described in more detail in theabove-mentioned International Patent Application WO 03/088303, themaximum rate of reciprocation of the scan arm 21 along its longitudinalaxis may be of the order of 1 Hz. For a given ion beam current, or fluxof the desired atomic species to be implanted, the time during which thesemi-conductor wafer can be exposed to the ion beam during the ionimplanting process is dictated by the recipe dose of the atomic speciesto be implanted. Assuming that over-scanning of the wafer relative theion beam is limited to the amount necessary to ensure an evendistribution of dose over the wafer, i.e. a square raster pattern suchas illustrated in FIG. 2, it can be seen that the limited total time forthe implant in combination with the limited mechanical scan speeddictates the spacing for the raster lines drawn by the scans over thewafer surface, assuming each scan draws a separate line.

Clearly, in order to ensure adequate uniformity of dose delivered to thewafer in the direction of the raster line spacing, this spacing or linepitch must be less than the cross-sectional dimension of the ion beam inthe line spacing direction (Y-coordinate direction of FIG. 1). Inpractice, the dose uniformity is improved with a smaller raster linepitch.

The recipe dose could be delivered to the wafer by performing a singlepass drawing a raster of scan lines at the line pitch dictated by thebeam current, the mechanical reciprocation rate of the scanningmechanism, and the required recipe dose. However, it may be preferableto deliver the same recipe dose to the wafer in multiple passes of thewafer across the ion beam, in order to reduce the thermal loading of thewafer caused by the impinging ion beam. Then, in order to maximise doseuniformity, the scan lines drawn during each pass are preferablyarranged to interleave scan lines of the previous pass to produce acomposite raster with a reduced line pitch.

For example, if the line pitch dictated by the recipe dose requirementsas explained above is T, then each of four passes could be made withscans separated by 4T. Each of the passes is arranged to spatially phaseshift the scans of the pass by the amount T, so that the compositeraster drawn by the four passes has lines with the pitch T. In this way,the thermal loading of the wafer is reduced whilst ensuring the rasterline pitch is maintained at the desired value T. FIG. 4 illustrates apart of the scanning pattern of four interleaved passes as describedabove.

A modified version of interleaved scanning is illustrated in FIGS. 13 a,13 b, 13 c and 13 d. In a known implant procedure, a desired implantdose is divided into equal parts with each part delivered to the waferwith the wafer at a different twist orientation. In the case of a zeroangle implant, with the wafer nominally normal to the ion beam, therequired recipe dose may be delivered in four equal parts, with theparts being delivered with the wafer at respective equally spacedorientations about an axis normal to the wafer plane (and parallel tothe ion beam). Thus, the different orientations for the four differentparts of the implant dose are spaced by 90° in this example. Theresulting implant procedure is sometimes referred to as a “quad”implant. One purpose of such quad implants is to cancel out the effectof any non-uniformity in the distribution of implant angles of the ionsin the beam.

In any such implant procedure where different parts of the implant doseare delivered with the wafer orientation twisted through 180°, each partof the dose may be delivered by a separate pass of the scanned waferthrough the beam, producing a distinct raster of scanned ions over thewafer. Then, rasters generated with the wafer twisted through 180° canbe interleaved in order to improve dose uniformity.

FIG. 13 a illustrates a first raster of scan lines 50 having a scanpitch of 2T. This first raster is performed with the wafer 51 in a firstorientation indicated by the position of arrow 52. Before performing asecond raster of scan lines, the wafer is re-oriented by 180° to theposition shown in FIG. 13 b. A second raster of scan lines is thenperformed with the lines 53 of a second raster interleaved midwaybetween the lines 50 of the first raster. The resulting composite of tworasters has an overall line pitch of T, so that the dose uniformity inthe direction perpendicular to the scan lines 50 and 53 can beincreased.

To complete the quad implant, two further rasters of scan lines 54 and55 are performed as illustrated in FIG. 13 c and FIG. 13 d, with thewafer orientated at 90° and 270° relative to the orientation of FIG. 13a. Again the scan lines 55 of the fourth raster are arranged to beinterleaved midway between the scan lines 54 of the third raster.

A quad implant of this kind can be performed by suitably programming thecontroller 31 in FIG. 1 to perform four separate passes of the waferthrough the beam, and operating a rotation drive motor 56 to re-orientthe wafer holder 18 as required between each pass.

In the above described example, the quad implant is performed with azero implant angle, so that the normal to the wafer is parallel to theion beam. However, if the implant is performed with the wafer tilted,e.g. to provide an angled implant as described above, it may sometimesbe desirable to adjust the orientation of the wafer for the differentsegments of the implant, not about an axis normal to the wafer, butabout an axis parallel to the ion beam. In this way, the angle of thebeam relative to the crystal axes of the wafer remains the same for thedifferent segments of the implant.

Although a “quad” implant is described in the example above, the sameinterleaving of rasters with the wafer rotated by 180° can be performedin higher order implants with the wafer re-orientated at 60° angularintervals (a “sextal” implant), 45° intervals (an “octal” implant), orany other angular interval which is an even factor of 360°.

In particular, a “dual” implant may be performed, in which the totaldesired implant dose is divided into just two parts, a first part beingdelivered with the wafer at a first twist orientation and a second partdelivered with the wafer twisted through 180° relative to the firsttwist orientation. In the specific example illustrated in FIG. 13, thiswould correspond to delivering half the total implant dose with thewafer orientated as in FIG. 13 a and the rest of the dose with the waferoriented as in FIG. 13 b.

The inventors have determined that a dual implant as described above canprovide all the advantages of a quad implant, when implanting atnominally zero implant angle, using an implanter which mechanicallyscans the wafer in a fully planar fashion through an ion beam which iscross-sectionally smaller than the wafer in both orthogonal directionse.g. a pencil ion beam. Then the principal or only purpose in performinga quad implant is to average out any beam angle alignment errors whichcan arise over successive implants, due to variations in day-to-daysetup or wafer crystal orientation.

Any such angle alignment error for a zero angle implant can be expressedin angular co-ordinates (θ, φ), where θ is angular deviation in a beamcontaining plane at angle φ. Importantly, reversing the twistorientation of the wafer by 180° for the second half of the dose, iseffective to average out the angular deviation θ, (between −θ to +θ)relative to the wafer, irrespective of the value of φ.

Since, for advanced devices, most high current implants are nowadaysdone at zero implant angle, performing a dual implant as describedabove, to achieve the same benefits as a quad implant, has significantadvantages. Single wafer implanters which mechanically scan a wafer intwo dimensions through a fixed ion beam mechanically reciprocate thewafer back and forth through the beam in a fast scan direction, whileprogressively translating the wafer in a slow scan direction to completea pass forming a raster of fast scans covering the wafer.

In order to achieve a desired dose uniformly on the wafer in the slowscan direction, transverse to the raster lines of the fast scan, thescanning should be done with a pitch between successive scan lines inthe raster, which is sufficiently small compared to the width of the ionbeam in the slow scan direction. This usually sets a minimum scan linepitch to achieve required dose uniformity, implying a minimum number ofscan lines in a pass over the wafer. Practical mechanical scanners havea maximum fast scan frequency, so that a predetermined minimum time isrequired to complete a single pass comprising the above minimum numberof scans.

Clearly therefore, in the absence of any interlacing of scan lines fromdifferent passes, a dual implant requiring two passes can take half thetotal time of a quad implant requiring four passes.

If the scan lines of passes at 180° opposite orientation are interlaced,as described above with reference to FIG. 13, each pass may have halfthe number of scan lines for the same dose uniformity. However a dualimplant with interlace can still take only half the time of a quadimplant with interlace.

In summary, significant advantages arise from performing a dual implantrather than a quad implant with an implant which mechanically scans awafer in two-dimensions through a fixed beam and which scans in thewafer plane, whenever the implanter has a minimum practical duration fora single two-dimensional pass of the wafer through the beam.

The dual implant method described above can be performed by an implanterwhich scans the wafer in a rectilinear fashion with parallel fast scansseparated by steps in the slow scan direction, or with zig-zag like fastscans formed with continuous movement in the slow scan direction.Further, the fast scan and/or the slow scan directions of movement neednot be linear, but may comprise arcs of a circle or other curve.

In addition to, or separately from, the above described advantages of adual implant, there are advantages in performing a 2-D mechanicalscan-type implant with a 180° twist in wafer orientation between eachraster or slow pass through the ion beam. With 2-D mechanical scanning,the passes of a wafer back and forth through the ion beam in thedirection transverse to the raster scan lines are relatively slow, dueto mechanical constraints. By reversing the orientation of the waferbefore each slow pass through the beam, excessive thermal loading of theleading edges of the wafer during the passes in each direction isavoided. Such excessive thermal loading would otherwise occur because,without a 180° twist, the leading edge of a wafer starting a new pass isalso the last part of the wafer to leave the ion beam at the end of thepreceding pass.

The above thermal loading advantages can be obtained, at least in part,even if the alternating passes of the wafer do not deliver the samedose, i.e. passes in one direction deliver more dose than passes in theother direction, or if the dose delivered varies over each pass.

In the examples described above and illustrated in FIGS. 13 a to 13 d,the spatial phase shift between the interleaved rasters is 50% of thepitch of each individual raster, assuming the scan lines shown in FIGS.13 a to 13 d represent the locus of the centre of the beam. This is finefor a beam having a profile which is symmetric in the Y, or scan linespacing, direction. However, the implant beam is commonly somewhatasymmetric. With a beam which is asymmetric in the Y-direction, andassuming the beam centre in the Y-direction is the mid point of thebeam, rasters interleaved with a phase shift of 50% of the pitch on awafer which has been re-orientated or twisted by 180°, can result in anon-uniformity of dose in the Y-direction which is significantly worsethan would be obtained when performing the two interleaved rasters on awafer held at a constant orientation.

The reason for this can be understood from the following analysis. Inthe case of a symmetric beam (in the Y-direction), re-orientating thewafer by 180° between interleaved rasters makes no difference to thedistribution (or the “footprint”) on the wafer of beam flux in theY-direction from the scan lines of the two rasters. Therefore thesymmetrical footprint of the Y-profile of the beam on the waferresulting from the interleaved rasters is repeated identically with aneffective spacing T/2 equal to half the pitch T of each of the rasters(as illustrated in FIG. 13 b).

However, for a beam which has an asymmetric profile in the Y-direction,the corresponding asymmetric footprint of the Y-profile of the beam onthe wafer is reversed during the second raster. As a result, identicalfootprints of the Y-profile of the beam on the wafer are repeated in theinterleaved rasters with an effective spacing equal only to the pitch Tof each of the rasters. The asymmetric features of the Y-profile have aneffective spacing T, whereas symmetric features would have an effectivespacing T/2. Since uniformity strongly correlates with the effectivespacing, the resulting uniformity for a beam which is asymmetric in theY-direction is worse.

This degradation of uniformity for interleaved scans with 180° wafertwist can be almost completely eliminated by selecting a phase shiftbetween the interleaved rasters different from T/2 (50% of the pitch ofeach raster). An optimum phase shift, different from T/2, can bedetermined from the measured Y-profile of the beam and the intendedpitch T of each raster.

A numerical analysis is performed using a computer to calculate the dosedistribution in the Y-direction from a single raster of line scans at apitch T. Then, this dose distribution is numerically superimposed on itsmirror image at different spatial phase shifts between the two and theresulting combined dose distributions calculated for the different phaseshifts in order to find the optimized phase shift providing the minimumdose variation in the Y-direction for the particular pitch value T.

Calculating the optimized spatial phase shift values over a range ofsingle raster pitch values T for a typical asymmetric beam profile showsthat a dose uniformity can be obtained which not only matches that forthe asymmetric beam interleaved at 50% without 180° wafer twist (same asa single raster at pitch T/2), but can provide at some values of T asignificantly better uniformity. This is illustrated in FIGS. 15, 16 and17.

FIG. 15 is a graphical representation of a somewhat asymmetric beamprofile in the Y-direction. FIG. 16 shows plots of non-uniformity ofdose (y-axis) against the pitch (x-axis) of scan lines for a raster ofparallel scans on a wafer, using the beam profile of FIG. 15. In FIG.16, line 70 is the non-uniformity from a single raster at variouspitches for a zero angle implant. Line 71 is the non-uniformity of twosuperimposed rasters, each having the same pitch as the single raster ofline 70 and spatially phase shifted by 50%, without any re-orientationof the wafer between the rasters. Therefore the plot of line 71corresponds to the plot of line 70 shifted to the right by doubling thepitch values on the x-axis.

Plot line 72 in FIG. 16 is the non-uniformity of the two superimposedrasters spatially phase shifted by 50%, but with the second rasterapplied to the wafer after 180° twist. The 50% raster shift is appliedto the scan line locus of the centre of the asymmetric beam in theY-direction.

By comparison, plot line 73 in FIG. 17 is the uniformity of the rasterssuperimposed with 180° twist and with raster shift optimized for eachvalue of pitch. As can be seen the uniformity obtained at certain lowpitch values is actually better than that obtainable without 180° twist.This improvement is counter-intuitive. However, it can be understoodfollowing realization that combining an asymmetric profile with itsmirror image at an optimised pitch shift can produce a combined profilewhich is both symmetrical and also wider than the original asymmetricprofile. The uniformity of dose achievable for scan lines at a selectedpitch is strongly correlated with the effective beam profile width inthe scan line spacing direction. Therefore, even though the rasterformed by the combined profile has a pitch (between the footprints onthe wafer of the composite lines formed by the combined profile) whichis equal to the actual pitch T of each of the separate rasters of beamscan lines, uniformity can be good because the combined profile issymmetrical and can have an effective width rather larger than theactual width of the ion beam profile.

In the examples of rasters of scan lines described above, the overallraster is substantially square in outline, even though the wafer to beimplanted is circular. If it is required to improve productivity andreduce the amount of time during each raster that the beam is passingthe wafer so that no beam flux is being absorbed, the scan lines can beadjusted in length to minimize the amount of time that no beam flux isbeing absorbed on the wafer. A modified raster is illustrated in FIG. 14in which the relative movement of the beam over the wafer is illustratedby lines 60, and the dotted lines 61 illustrate those parts of therectangular raster which are left out of the modified scan pattern. As aresult, the time to produce the raster illustrated in FIG. 14 is reducedand productivity of the implanter can be increased. Throughput can beincreased by 10-20%. The scan pattern may not be completely circular asillustrated in FIG. 14. For example, if the beam profile is measured tobe asymmetric, the scan lines may extend further beyond the wafer edgeon one side than the other. Further, if an angled implant is performed,the outline shape of the scan pattern in a plane perpendicular to thebeam direction is modified accordingly and may become elliptical. Also,it can be desirable for dosimetry reasons to permit the entire beamcurrent to bypass the wafer and holder and enter the beam stop Faradayfor longer periods at some time or times during an implant. A suitablescan pattern outline might therefore be substantially circular but withone complete square corner. As before, FIG. 14 for convenienceillustrates the way the beam scans over the wafer, whereas in amechanically scanned apparatus as illustrated in FIG. 1, it is in factthe wafer which is scanned over a stationary beam.

Although the raster line spacing which can be used with a mechanicalscanning system is dictated by the recipe dose and beam current, asexplained above, it is nevertheless important to ensure that the linespacing or pitch is small enough to provide the desired dose uniformityin the spacing direction. This uniformity is a function of the shape ofthe beam profile and the raster line pitch. A smaller pitch results in amore uniform dose distribution over the wafer. The following is amathematical description of how the uniformity varies as a function ofscan pitch for a parallel line scanning pattern. Fourier transformationis used as a tool since the analysis involves periodic patterns in dosedistribution.

In this study, it is assumed that beam current and profile do not changeduring an implant. Time dependency is not treated, although that is verysimilar. Notations τ, t, u, ν and T are used as spatial variables, andlikewise, ω and 1/T as spatial frequencies.

In a parallel line scanner, implant dose h(t) at a location t on a wafercan be written using a periodic δ-function as: $\begin{matrix}\begin{matrix}{{{h(t)} = {\int_{0}^{1}{{{b(\tau)} \cdot {\delta_{T}\left( {t - \tau} \right)}}\quad{\mathbb{d}\tau}}}},} \\\left( {{\delta_{T}\left( {t - \tau} \right)} = {\sum\limits_{n = {- \infty}}^{\infty}\quad{\delta\left( {t - \tau - {n\quad T}} \right)}}} \right)\end{matrix} & \left. 1 \right)\end{matrix}$where b(τ) is the beam profile in the slow scan direction (y in FIG. 2)and δ_(T)(τ) is a periodic δ-function of period T. T corresponds to onetranslation step or the spacing between raster lines in a compositeraster. FIG. 5 illustrates the functions and FIG. 6 illustrates theraster scanning pattern. The wafer 17 is scanned in front of the beam 19in parallel lines spaced apart by a constant distance T. For simplicity,b(τ) is defined within a range [0, 1]. In other words, the step T isnormalized to the actual beam size. Equation 1 gives the accumulateddose at a location t after one complete slow scan or one complete rasterwith 1/T fast scans. Dose h(t) is also a periodic function with a periodT.

Fourier transforms of the functions of Equation 1 are defined asfollows: ${b(t)}\overset{\quad{FT}\quad}{\rightarrow}{B(\omega)}$${\delta_{T}(t)}\overset{\quad{FT}\quad}{\rightarrow}{\Delta_{\omega_{0}}(\omega)}$${h(t)}\overset{\quad{FT}\quad}{\rightarrow}{H(\omega)}$

The Fourier transform of a periodic δ-function is also a periodicδ-function. $\begin{matrix}\begin{matrix}{{{\Delta_{\omega_{0}}(\omega)} = {\omega_{0} \cdot {\sum\limits_{n = {- \infty}}^{\infty}\quad{\delta\left( {\omega - {n\quad\omega_{0}}} \right)}}}},} \\{\omega_{0} = {2\quad{\pi/T}}}\end{matrix} & \left. 2 \right)\end{matrix}$

Using the convolution theorem and notations above, Equation 1 istransformed as: $\begin{matrix}{{H(\omega)} = {{{B(\omega)}{\Delta_{\omega_{0}}(\omega)}} = {{\omega_{0} \cdot {B(\omega)} \cdot {\sum\limits_{n = {- \infty}}^{\infty}\quad{\delta\left( {\omega - {n\quad\omega_{0}}} \right)}}} = {\omega_{0} \cdot {\sum\limits_{n = {- \infty}}^{\infty}\quad{{B\left( {n\quad\omega_{0}} \right)} \cdot {\delta\left( {\omega - {n\quad\omega_{0}}} \right)}}}}}}} & \left. 3 \right)\end{matrix}$

This is a significant result. Equation 3 shows that the Fouriertransform of dose h(t) consists of a series of impulses with a constantinterval ω_(o)=2π/T, and that the amplitude of each impulse at ω=nω_(o)is equal to ω_(o)B(nω_(o)), that is ω_(o) times the spectral density ofbeam profile at ω=nω₀.

Variance, σ², of the implant is defined as $\begin{matrix}{\sigma^{2} = {{1/T} \cdot {\int_{0}^{T}{\left( {{h(t)} - \overset{\_}{h(t)}} \right)^{2}\quad{\mathbb{d}t}}}}} & \left. 4 \right)\end{matrix}$where h(t) is the average dose. The integration is performed in theregion of [0, T] since h(t) is a periodic function of period T.$\begin{matrix}{\overset{\_}{h(t)} = {{{1/T} \cdot {\int_{0}^{T}{{h(t)}\quad{\mathbb{d}t}}}} = {{\frac{1}{2\quad\pi}{H(0)}} = {\frac{\omega_{0}}{2\quad\pi} \cdot {B(0)}}}}} & \left. 5 \right)\end{matrix}$

If the dose deviation is written as${{f(t)}\overset{\quad{FT}\quad}{\rightarrow}{F(\omega)}},$then from Equation 4, σ² = 1/T ⋅ ∫₀^(T)f(t)²  𝕕t

This is the definition of the power of f(t). Using Parseval's theoremthis can be expressed as:$\sigma^{2} = {\sum\limits_{n = {- \infty}}^{\infty}{F_{n}}^{2}}$where |_(F) _(n) | is the amplitude of the n-th term of Fourier series.

From Equations 3 and 5, $\begin{matrix}\begin{matrix}{{F(\omega)} = {{{H(\omega)} - {H(0)}} = {{\omega_{0} \cdot {\sum\limits_{n = {- \infty}}^{\infty}{{B\left( {n\quad\omega_{0}} \right)} \cdot {\delta\left( {\omega - {n\quad\omega_{0}}} \right)}}}} - {\omega_{0} \cdot {B(0)} \cdot {\delta(\omega)}}}}} \\{= {{\omega_{0} \cdot {\sum\limits_{n = {- \infty}}^{- 1}{{B\left( {n\quad\omega_{0}} \right)} \cdot {\delta\left( {\omega - {n\quad\omega_{0}}} \right)}}}} + {\omega_{0} \cdot {\sum\limits_{n = 1}^{\infty}{{B\left( {n\quad\omega_{0}} \right)} \cdot {\delta\left( {\omega - {n\quad\omega_{0}}} \right)}}}}}}\end{matrix} & \quad \\{{F_{n} = {{\frac{1}{2\pi}{F\left( {n\quad\omega_{0}} \right)}} = {\frac{\varpi_{0}}{2\pi}{B\left( {n\quad\omega_{0}} \right)}}}},\left( {{n = {\ldots - 2}},{- 1},1,2,\ldots} \right)} & \quad\end{matrix}$

Since b(t) is real, |B(nω₀)|=|B|(−nω₀)|.Therefore,$\sigma^{2} = {2 \cdot \frac{\omega_{0}^{2}}{4\quad\pi^{2}} \cdot {\sum\limits_{n = 1}^{\infty}\quad{{B\left( {n\quad\omega_{0}} \right)}}^{2}}}$and the standard deviation, σ is $\begin{matrix}{\sigma = {\frac{\omega_{0}}{2\pi} \cdot \sqrt{2 \cdot {\sum\limits_{n = 1}^{\infty}{{B\left( {n\quad\omega_{0}} \right)}}^{2}}}}} & \left. 6 \right)\end{matrix}$

The relative standard deviation, σ_(r), normalized to the average dose,h(t) (see Equation 5), is $\begin{matrix}{\sigma_{r} = {{\sigma/\overset{\_}{h(t)}} = {\sqrt{2 \cdot {\sum\limits_{n = 1}^{\infty}{{B\left( {n\quad\omega_{0}} \right.}^{2}}}}/{B(0)}}}} & \left. 7 \right)\end{matrix}$

Equations 6 and 7 describe how dose non-uniformity depends on ω₀, thuson the slow scan step T. The standard deviation of the dose implanted ata frequency 1/T consists of the amplitudes of B(nω₀), (ω₀=2π/T,n=1, 2, .. . ) in the spectral density of the beam function. σ does not containan amplitude B(ω) for ω≠nω₀ nor for ω=0. Equations 6 and 7 aremathematically accurate only when the infinite summation is performed.However, B(ω) is usually a fast diminishing function of ω, and a fairlygood approximation of σ and σ_(r) is obtained by summing only a fewlower harmonic terms in the series.

Calculating B(ω) in the continuous frequency domain is time consuming.Instead, one can use Fast Fourier transform (FFT) and derive values ofB(ω_(n)) for the discrete frequencies, ω_(n)=2π(n=0, 1, 2, . . . ). FFTof a beam profile that consists of 2^(I) data points yields B(ω) only upto ω_(I-1)=2π·2^(I-1). If the beam profile function is measured andknown as a vector each element of which corresponds to b(Δτ·m), m=0, 1,2, . . . , 2⁷), for instance, one can achieve B(ω) up to a frequency of2⁷⁻¹=64 Hz. Equations 6 and 7 then provide fairly accurate values of σand σ_(r), respectively, up to a frequency of 2⁷⁻²=32 Hz. B(ω) can bederived at non-integer frequencies by using the scaling property of theFourier transform. With α>1 being a scaling factor, the Fouriertransform of a function b(αt) yields B(ω)/α for a series of frequencies,ω_(n)=2n π/α(n=0, 1, 2, . . . ).

FIG. 7 shows the calculated σ_(r) as a function of frequency, 1/T, forthe arbitrary beam profile function shown in FIG. 5. FFT is performedfor b(Δτ.m) up to a length 2⁷. A typical requirement is for σ_(r) to besmaller than 0.5% after an implant.

From the graph, one can choose 1/T>8 as the implant scan spatialfrequency. Thus the slow scan step or normalised scanning raster linespacing T should be less than ⅛th of the beam dimension in the spacingdirection (Y) to meet the required dose uniformity σ_(r)<0.5%.

In FIG. 8, a plot of σ_(r) is shown after applying the scaling propertyto B(ω) with two different values of α.

In FIG. 9, the FFT result is compared to that of exact calculation byEquation 4. The FFT agrees with the exact curve well despite that thebeam profile b(t) was sampled at only 2⁷=128 points. It may be noticedthat σ_(r) has a periodic pattern in addition to a fast diminishingtrend as frequency rises.

If the beam profile function is a Gaussian, as illustrated in FIG. 10,σ_(r) diminishes very quickly as scan pitch decreases. This is becauseFourier transform of a Gaussian function is also a Gaussian, as shownbelow: $\begin{matrix}{{b(t)} = {{{\exp\left( {{- \alpha^{2}}t^{2}} \right)}\overset{\quad{FT}\quad}{\rightarrow}{B(\omega)}} = {\left( {\sqrt{\pi}/\alpha} \right) \cdot {\exp\left( {{{- \omega^{2}}/4}\alpha^{2}} \right)}}}} & \left. 8 \right)\end{matrix}$

It can be seen that σ approaches 0 fast due to the term|B(nω₀)|²=(π/·α²)·exp(−nω₀ ²/2α²). A smaller value a in Equation 8 givesa broader profile and faster decrease in σ.

The graph shown in FIG. 11 illustrates the case where a=5.7. σ_(r) fallsbelow 0.5% at 1/T=4.3 Hz. This is almost half the frequency of thearbitrary beam profile of FIG. 5.

Summarising the above, a fast Fourier transform method can be used tocalculate the variation in dose uniformity across the wafer in theraster line spacing direction with raster line pitch, for a particularbeam profile function in the spacing direction. In operation, the abovecalculations can be performed quickly by a computer, such as a computerused for other implanter control functions. Known FFT processors orsoftware applications may be used for this purpose. Instead of FFTtechniques, the uniformity calculations may be performed in other ways,such as by use of computer simulation.

When performing an implant using the above-described scanning process,the value for the raster line spacing of the scanning is selected whichis sufficiently small relative to the beam dimension in the spacingdirection to ensure that the dose uniformity in this direction meets therequired standard, <0.5% in the above-described examples.

A number of different procedures may be used for determining the profilefunction of the ion beam in the raster line spacing direction for use inthe above calculations of dose uniformity. For example, the arrangementdisclosed in the above-referred U.S. patent application Ser. No.10/119290 may be employed. In this arrangement a small Faraday having anopening of typically 1 cm², is mounted on the scanning arm 21, so thatit can be scanned across the ion beam by appropriate operation of themotors 25 and 26 to drive the scanning arm 21 in its two componentdirections. A full two-dimensional map can then be made of the beamcross-sectional profile, in both X- and Y-directions and the effectiveprofile function in the Y-direction can be calculated.

Using the measured profile function in the Y-direction, the maximumvalue can be calculated as outlined above of raster line spacing orpitch which can provide the desired dose uniformity. When performing theimplant, the scanning mechanism is then controlled to ensure that theraster line spacing employed does not exceed the calculated maximum.

In practice, it is often desirable to use a raster line spacing for thescanning which has been calculated from the beam current available, thedose of ions to be implanted per unit area of wafer, according to theimplant recipe, and the speed at which the scanning mechanism cancomplete the desired number of passes across the beam (which is in turndependent on the maximum scan speed in the X-direction. Accordingly,before performing an implant, this desired value of the raster linespacing is calculated, having measured the beam cross-sectional profileand the beam current available. Then, this desired line spacing iscompared with the calculated variation of dose uniformity obtained fordifferent line spacings to ensure that the calculated desired spacingvalue will give the required dose uniformity. The implant proceeds onlyif the calculated spacing value provides satisfactory uniformity.

If the calculated uniformity using the desired spacing value is worsethan the desired uniformity, the beam flux may be reduced. Reducing thebeam flux results in the calculated line spacing also being reduced. Thebeam flux is reduced to a level at which the calculated dose uniformitymeets the required standard. The beam flux may be reduced by adjustingone or more of the operating parameters of the ion source including therate of supply of the feed gas to the arc chamber of the source, the arcsupply power, the cathode power, the extraction voltage and the spacingof the extraction electrodes from the ion source. Appropriateadjustments will be known by the skilled person. Alternatively, beamflux can be adjusted by altering a beam aperture at a selected locationalong the beam path, so as to reduce the current of beam passing throughthe aperture.

Alternatively, or as well, the calculated uniformity can be improved bymodifying the beam profile. As demonstrated above, if the beam profileis closer to a Gaussian shape, the blending properties of the beam canbe substantially improved so that good dose uniformity is obtainablewith larger raster line spacings. Retuning of the beam, in ways whichare well understood by the workers in this field, can improve the beamprofile. Retuning can minimize clipping of the beam as it passes fromion source to process chamber, and thereby remove undesirableasymmetries or peaks in the beam profile. The beam could be retuned, forexample, by adjusting the lateral position of ion source extractionelectrodes relative to the arc chamber of the source, to ensure theextracted beam is directed centrally along the optimum flight path alongthe beam line of the implanter. Beam profile may also be modified byintroducing a single magnetic quadrupole in the beam line to produce adesired spreading of the beam in the Y-direction.

In this procedure, the beam profile is remeasured and the resultinguniformity using the desired line spacing is recalculated with a view tomeeting the required uniformity specification.

In the examples of the invention described above, the scanning mechanismis controlled to provide a stepped slow scan, with a uniform stepmovement between successive fast scans, to produce parallel scan lineson the wafer. In other embodiments of the invention, a continuous slowscan action may be employed with simultaneous fast scans, to produce azig-zag like scan pattern on the wafer, e.g. as illustrated in FIG. 12.Tolerable dose uniformity in the slow scan direction 40 can still beobtained for example if the mid-points 41, 42 of successive scans areuniformly spaced in the slow scan direction. It can be seen however thatdose uniformity is increasingly compromised towards the edges of thewafer 43 on either side of the centre line 44 where the spacing betweensuccessive line pairs becomes increasingly different. Nevertheless, ifthe pitch (between line mid-points) is sufficiently small compared tothe beam size in the slow scan direction, adequate uniformity over thewhole wafer can be obtained. It is still important to control the slowscan speed relative to the fast scan repetition rate to achieve a linemid-point spacing which is small enough to provide the desireduniformity. Dose uniformity in the slow scan direction can be calculatedat locations near the edge of the wafer where uniformity will be mostcompromised by the zig-zag scan pattern. These calculations can beperformed using Fourier transform techniques similar to those discussedabove for parallel line scanning. Alternatively, computer simulationtechniques may be employed.

In order to ensure good overall uniformity in the slow scan direction, aminimum pitch size (between line mid-points) is determined bycalculation from knowledge of the beam profile shape. Then subsequentscanning of the wafer must maintain a pitch size no greater than thiscalculated minimum.

As with parallel line scanning, it may be necessary to reduce beamcurrent or improve beam profile, to achieve the required minimum pitchsize without exceeding the required recipe dose on the wafer.

To compensate for any variation in beam current during a pass, this ismeasured at turnaround points between successive fast scans and thespeed of subsequent fast scans controlled accordingly. The repetitionrate of fast scans is preferably maintained constant, so that changes infast scan speed do not effect the pitch between scan line mid-points.This may require some dead time available at each fast scan turnaroundto accommodate the increased fast scan time consequent on a reduced fastscan speed for example.

In the above-described examples, the mechanical system disclosed isbased on the structure described in the above-referred InternationalPatent Application WO 03/088303. Any other form of mechanical scanningarrangement may also be used to provide the desired equally spaced andparallel raster lines. For example, the articulated arm scanningmechanism disclosed in United Kingdom Application No. 0214384.0 filed 21Jun. 2002 may be employed.

Also, the scans of the substrate holder forming the parallel rasterlines need not be precisely linear but could, for example, be formed asarcs of concentric circles.

Many other arrangements may be employed for providing the essentialfeatures of the invention as set out in the following claims.

1. A method of implanting a substrate wafer having an implant surfacewith a predetermined diameter, the method comprising: a) generating animplant beam having a predetermined beam direction, b) holding saidsubstrate wafer aligned in an implant plane, normal to said beamdirection, c) said beam having orthogonal cross-sectional dimensions insaid implant plane each of which is less than said predetermined waferdiameter, and d) mechanically scanning said substrate wafer in twodimensions in said implant plane to provide a desired uniform totalimplant dose over the implant surface of the substrate wafer, whereinsaid mechanical scanning is performed so as to deliver said totalimplant dose in first and second parts, said first part being deliveredby performing said mechanical scanning with said substrate wafer held ina first orientation about a twist axis parallel to said predeterminedbeam direction, and said second part being delivered to complete saidtotal implant dose by performing said mechanical scanning with thesubstrate wafer held in a second orientation at 180° to said firstorientation about said twist axis.
 2. A method as claimed in claim 1,wherein said mechanical scanning is performed so that each of said firstand second parts of said total implant dose are delivered evenly overthe implant surface of said substrate wafer.
 3. A method as claimed inclaim 2 wherein said first and second implant dose parts are equal.
 4. Amethod as claimed in claim 1 wherein the substrate wafer is mechanicallyscanned so that, during delivery of each of said first and secondimplant dose parts, the implant beam draws on the implant surface of thesubstrate wafer at least one raster of scan lines.
 5. A method asclaimed in claim 4, wherein the substrate wafer is mechanically scannedso that the implant beam draws on the implant surface of the substratewafer successive said rasters with the substrate wafer held forrespective said successive rasters alternately in said first and saidsecond orientations.
 6. A method as claimed in claim 4, wherein eachsaid raster comprises uniformly spaced parallel scan lines and the scanlines drawn in one said raster of said second implant dose part areinterlaced with the scan lines drawn in preceding said raster of saidfirst implant dose part.
 7. A method as claimed in claim 6, wherein saidrasters of interlaced scan lines have a spatial phasing which isselected to optimize uniformity of the total implant dose.
 8. A methodas claimed in claim 7, wherein said spatial phasing of said rasters isselected by measuring the intensity profile of the implant beam in saidimplant plane in a direction transverse to said scan lines and usingsaid beam profile to determine a spatial phasing providing optimum totaldose uniformity.
 9. A method of implanting a substrate wafer having animplant surface with a predetermined diameter, the method comprising: a)generating an implant beam having a predetermined beam direction, b)holding said substrate wafer aligned in an implant plane, normal to saidbeam direction, c) said beam having orthogonal cross-sectionaldimensions in said implant plane each of which is less than saidpredetermined wafer diameter, and d) mechanically scanning saidsubstrate wafer in two dimensions in said implant plane to provide adesired uniform total implant dose over the implant surface of thesubstrate wafer, wherein the substrate wafer is mechanically scanned sothat the implant beam draws on the implant surface of the substratewafer alternating first and second rasters of scan lines, each of saidfirst and second rasters covering the whole of said implant surface,said first rasters are drawn with said substrate wafer held in a firstorientation about a twist axis parallel to said predetermined beamdirection, and said second rasters are drawn with the substrate waferheld in a second orientation at 180° to said first orientation aboutsaid twist axis.